The present invention relates generally to machine tools, more particularly relates to laser-equipped machine tools for cutting metal sheet and plate and specifically relates to controlling a laser apparatus to pierce and cut metal sheet and plate of various types and thicknesses.
A heavy-duty laser equipped Computer Numerical Controlled (CNC) machine tool has been developed for cutting steel, stainless steel, aluminum and other metal sheets and plates within the range of approximately 0.040 to 1.25 inches thick. The machine has been adapted with a 6 kW laser to cut metal plate, and with linear motors to quickly cut thin metal sheet. A flexible means of controlling a piercing cycle and transitioning to a cut is needed for such a machine.
It is advantageous that the piercing, transition and cutting parameters are associated with a particular machine tool rather than being dependant upon a separate programming means. All of a plurality of identical laser equipped machine tools may not cut exactly the same. Differences in age, run time, and time between service intervals can cause laser cutting machines to have different cutting characteristics. When piercing, transition and cutting parameters are associated with or comprise part of a unique machine tool, it is possible to move a part program to any of a plurality of identical machine tools, run it there without change and achieve good results.
It is also advantageous to improve the process of piercing a plate to reduce the piercing time and to minimize the residual heat left in the plate by the piercing process. Reducing the piercing time improves productivity of the machine tool. Minimizing the residual heat left in the plate by the piercing cycle improves the cutting characteristics of the workpiece.
When a laser beam is applied to cut a workpiece, a piercing operation must be performed at the start of the cut. Piercing thinner metal sheets with a laser is a well-established and conventional process. The piercing operation is typically performed with the output of the laser maintained constant, that is, with the continuous wave power level or pulsed beam power level, pulse frequency, and duty cycle maintained constant from start to end of piercing. Piercing thicker metals is difficult and slow with this process, particularly when piercing carbon steel plate with oxygen assist gas. The piercing time for a one-inch thick carbon steel plate utilizing this process, a 6 kW laser, and a cutting head, in which the focal lens position is fixed relative to the cutting nozzle during automatic operation, may be over one minute.
Nakata et al. U.S. Pat. No. 5,434,383 discloses an improved piercing process in which the pierce is started with an initial pulse frequency and an initial pulse duty ratio, and the pulse frequency and the pulse duty ratio are increased by predetermined increments over predetermined time periods. Column 1, lines 12-20, describes piercing thick carbon steel as extremely difficult. The disclosed method of programming a pulse frequency increment and a pulse duty ratio increment fixes the rate of change of the frequency and the duty ratio such that it is linear over the total time period. One of the inventors of the present invention observed operation of a machine tool equipped with a 6 kW laser. The machine tool was equipped with a cutting head in which the focal lens position was fixed relative to the cutting nozzle during automatic operation. The time of piercing a one-inch thick carbon steel plate using the technique disclosed in U.S. Pat. No. 5,434,383 ranged approximately from 20 to 60 seconds. Variations in chemistry and/or surface characteristics of the material cause the great variation in pierce time.
A defocused pierce process is another method for reducing the piercing time of a metal plate. The process is described in Kanaoka et al. U.S. Pat. No. 5,770,833 (though not called therein xe2x80x9cdefocused pierce processxe2x80x9d) at column 2, lines 6-20, with reference to FIG. 15. Column 2, lines 21-54 describe problems associated with the process. FIGS. 16A, 16B and 16C of the patent are examples of the upper surface of 12 mm thick mild steel pierced with the process. Columns 2 and 3, lines 55-7 describe the process using a double nozzle. FIGS. 18A and 18B show the upper and lower surfaces of a 12 mm mild steel plate pierced with a double nozzle and the defocused pierce process. The diameter of the pierced hole produced by the double nozzle is approximately twice that of the holes pierced with a conventional or single nozzle.
Carbon steel plate in the range of 0.75 to 1 inch thick can be pierced in 3 to 5 seconds with the defocused pierce process and a 6 kW laser. However, the process is problematic in this range of thickness. The pierced hole is approximately 0.25 to 0.375 inches in diameter. So much material is blown upward that the nozzle and the focal lens are often damaged. The pierced plate is often left very hot impeding the following cutting process. When employing the defocused pierce process on a plate that has many pierced holes, it is advisable to pierce all the holes first then stop and sweep the surface of the plate clean before continuing with cutting the plate. Such requires operator intervention and thus impedes automatic and unattended operation of the machine tool.
Kanaoka et al. U.S. Pat. No. 5,770,833 discloses several improvements to the defocused piercing process. The improvements include the steps of: (1) locating a processing head at a piercing start position such that the laser beam is focused at a point spaced vertically from the surface of the workpiece and offset horizontally from the intended piercing point; and (2) moving, while irradiating with a laser beam and jetting an assist gas, the processing head simultaneously in directions parallel and perpendicular to the workpiece surface, to the piercing point. Further improvements involve the type and/or control of the assist gas.
Topkaya et al. U.S. Pat. No. 5,332,881 discloses a laser cutting head with an automatically adjustable optical focusing system. Column 2, lines 41-43 mentions drilling a workpiece by means of a laser beam. Lines 43-54 describe continuous displacement of the focal point in the direction of the beam during drilling as advantageous for thick workpieces.
The inventors have developed a flexible method and apparatus for controlling laser piercing of metal sheet and plate and transitioning to a cut. A controller is configured such that pierce and cut control parameters are associated with and comprise part of a unique laser equipped machine tool. A piercing cycle is subdivided into a number of sequential increments, and the time duration of each increment is individually selectable, as are the machine operating parameters. The controller is flexible such that a piercing cycle can be optimized for type of material, i.e. mild steel, alloy steel, stainless steel, aluminum, etc., and for material thickness. Further, the controller is adapted such that parameters controlling laser mode, power, pulsing characteristics, focal position, and assist gas pressure can be changed and auxiliary functions can be selectively engaged and disengaged within the pierce cycle.
Preferably the controller is resident in either the CNC of a laser equipped machine tool or another computer communicating with the CNC. The term xe2x80x9ccomputer systemxe2x80x9d will be used herein as a generic description of the multiple types of computer configurations found in machine tools of this type. The terms xe2x80x9cCNCxe2x80x9d and xe2x80x9ccomputer systemxe2x80x9d are inclusive of one or more of such systems connected in a computer network. In a preferred embodiment, the controller includes a Material Parameter Library which is comprised of a plurality of computer readable files. An individual file is herein called a Material Parameter Library file, a MPL file, or a MPL record. The Material Parameter Library can be considered a database and may be comprised as such. The Material Parameter Library resides in a directory, herein called a Material Parameter Library (MPL), a MPL directory, or a MPL database, of a computer that serves as or is associated with a CNC control (i.e., the computer system) controlling operation of a laser equipped machine tool. When the computer system is configured in this way, a part program, operating in the CNC, can operate on information from particular files in the Material Parameter Library and utilize the parameters stored in a particular MPL file or a plurality of particular MPL files to accomplish the process required by the part program. This is particularly significant in piercing of the workpiece, where the library can include a number of tables, which will be described below, which provide sets of parameters intended to be operated in sequence to control the laser machine operation to perform piercing optimized to the particular type of workpiece being operated on by the part program.
In greater detail, and according to a specific implementation of the invention, the Material Parameter Library contains a plurality of OEM MPL files and/or User MPL files wherein each file contains parameters to pierce and cut a specific type, alloy, and thickness of material. The pierce cycle of any file is subdivided into a plurality of sequential increments, with the time duration (and machine parameters) for each increment being individually selectable. Opened on an operator station of a laser-equipped CNC machine tool, the MPL file appears on a visual display screen and comprises part of the Human Machine Interface, HMI. An MPL file serves, in conjunction with a part program, the CNC control, a laser apparatus, a cutting head with related apparatuses, and cutting related auxiliary apparatuses, as a controller for controlling a pierce cycle for laser piercing metal sheet and plate and transitioning to a cut. An MPL file can be called from a part program or can be selected and loaded from the operator station.
The term OEM of OEM MPL file is used by example and not by limitation. The term OEM is intended as a means to identify MPL files that are prepared and furnished by the Original Equipment Manufacturer, OEM, the manufacturer of the laser-equipped machine tool. As such, an OEM MPL file can be identified by any unique term selected by the OEM. An OEM MPL file is a protected file that is xe2x80x9cread onlyxe2x80x9d to the machine tool owner/operator(s) and is updateable by the Original Equipment Manufacturer or is updateable by an owner/operator such that the OEM MPL file content is controlled by the OEM. Preferably, the computer storing the Material Parameter Library is adapted such that OEM MPL files are remotely updateable by the OEM. The time duration of the increments and the parameters are considered xe2x80x9cselectablexe2x80x9d in the broadest sense, even though in an OEM MPL file they are not directly selectable by the user. They are selected by the OEM and, with controls are made selectable by the user.
The term User of User MPL file is used by example and not by limitation. The term User is intended as a means to identify the creator of a User MPL file that is prepared by an owner or operator of the laser-equipped machine tool. As such the term User is typically the name, initials, nickname or other identifying term of the person that created the User MPL file.
An MPL file is adapted substantially as a table with fields that control a unique function or parameter of operation. OEM MPL files are furnished with the laser equipped machine tool as a starter set. OEM MPL files are developed with intent for robust operation. They have been proven to work well cutting a specific type and thickness of metal and typically have been tested on material from several suppliers.
A User MPL file is one developed for a type or thickness of material not included in the OEM Material Parameter Library or for a special circumstance. An operator creates a User MPL file by opening a New MPL file, or by copying an OEM or a User MPL file and renaming and saving it. The operator then develops the file by editing its parameter fields and saving the edited file.
It is a feature of the invention that a MPL Manager is provided to help a machine operator find a particular MPL file. The MPL Manager is adapted to conduct searches for MPL files, open, copy, or delete MPL files and to create new MPL files.
It is a feature of the invention that a pierce cycle is adapted as a plurality of variable time increment steps in which the duration of each step is selectable, in some cases by the OEM, and in other cases by the user. In the specific example described in the specification, though more or fewer steps could be adapted, the inventors have adapted nine, labeled T0 through T8. The duration of a step is established by entering a value in a table parameter field labeled xe2x80x9cTime Increment (sec)xe2x80x9d. Each step is associated with a plurality of parameter fields, each parameter field controlling a feature of the cycle.
T0 is a preparatory step. The T0 step sets the initial state or value of each parameter before subsequent steps are processed.
It is a feature of the invention that all steps do not have to be used. Setting the time increment of a step to 0.000 deactivates that and subsequent steps. The background color of deactivated steps is changed to show they are inactive.
It is a feature of the invention that the parameter fields of the deactivated steps do not have to be set to an inactive value. Parameter data in a deactivated step is ignored.
In a preferred embodiment of the invention, changed parameter values, other than on, off, and gas type are activated by linearly ramping the change across the time increment of the step. An alternate embodiment of the invention includes a parameter field settable such that a changed parameter value, other than on, off, and gas type, is alternatively changed at the beginning of the step time period or ramped across the step time period.
It is another feature of the invention, in transitioning from a pierce to a cut, that the axis controlling the position of the cutting nozzle relative to the surface of the workpiece can be held in last vertical position for a selectable distance into the cut. The vertical position can be temporarily held to prevent a nozzle position control system from reacting to any residue around the top of the pierced hole.
It is a further feature of the invention that a MPL file is optionally adapted with or is associated with one or more other control means also adapted substantially as a table(s) and controlling another aspect of a laser cutting process of a laser equipped machine tool.
In a preferred embodiment of the invention, the MPL file controls a cutting head that has a cutting nozzle that is adapted with a plate surface sensor preferably a capacitive sensor apparatus. References to a capacitive sensor herein are intended to broadly encompass plate surface sensors including those using non-capacitive sensing mechanisms. In practice the cutting head is positioned in the Z-axis by a servomotor. The capacitive sensor senses the distance between the cutting nozzle and the workpiece and provides a feedback signal to the Z-axis servo system to maintain the nozzle to workpiece distance at set value. Currently available capacitive sensor systems are inherently limited to a practical range of operation of approximately 0.004 to 0.400 inch.
In one example of use of the invention, in piercing plate it is sometimes advantageous to move the cutting nozzle to a position above the plate, called a standoff position, which is out of the operational range of the capacitive sensor, for example 0.500 inch. In practicing this example of the invention a second and alternate feedback apparatus is adapted to the servomotor that drives the Z-axis. The T0 step of the invention is adapted with an embedded cycle; Find Plate, for finding the workpiece. When the cutting head is moved from the end of a cut to a piercing position for the next cut it is often carried at a retracted position well above the workpiece to reduce risk of collision with an upturned part. Upon reaching the piercing position the pierce cycle is initiated. The T0 step first runs the Find Plate cycle. The Z-axis lowers the cutting head with the capacitive sensor active, senses the workpiece and positions the cutting nozzle to a standoff position that is set within the embedded cycle, i.e. 0.050 inch. Then the Z-axis feedback is switched from the capacitive sensor to the alternate feedback. Then the cutting head is raised positioning the cutting nozzle to the standoff position associated with the T0 step, 0.500 inch in this example. The entire pierce cycle is performed with the Z-axis controlled by the alternate feedback apparatus. At the transition to the cut, the alternate feedback remains in control of the standoff distance for a Z-axis hold distance. Upon cutting the Z-axis hold distance, the feedback is switched from the alternate feedback to the capacitive sensor for the remainder of the cut.
The invention also has method aspects. Particularly, the invention relates to a method of controlling piercing and a transition to cutting in a laser-equipped machine tool. A Material Parameter Library is provided and stores a plurality of sets of pierce cycle parameters for a plurality of material types. Pierce cycles are subdivided into a plurality of sequential increments and the duration of each increment, as well as the machine parameters operative for that increment, are separately specified for each increment. When a pierce cycle is required, such as by operation under a part program, information from a MPL record is accessed which relates to the type of material being processed. The record includes a sequence of sets of machine settings or other parameters (each set corresponding to one of the increments), and an operating time associated with each such set. The parameters in the sequential sets are established for optimized piercing of the type of material at issue. When the record is accessed, it serves to control the laser-equipped machine tool according to the sets of stored parameters in sequence for the stored operating time of each set in order to perform the pierce cycle. One of the parameters for piercing is configurable as a transition parameter, during which the machine parameter is maintained at the levels set in the record and under the control of the alternate position feedback mechanism so that the cut can be commenced without disturbance from any material surface imperfections around the pierced hole.
These and other objectives and features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.